Method and apparatus for continuous countercurrent pulping of ligno-cellulose materials



June 13, 1961 K. E. BRADWAY E'rAL 2,988,470

METHOD ANU APPARATUS FOR CONTINUOUS OOUNTERCURRENT PULPTNG OF LIGNO-CELLULOSR MATERIALS Filed May l2, 1959 y 2 Sheets-Sheet 1 cH/Ps i550 NECH//l//SM ,afgaan /A/E'T 2,988,470 ENT June 13, 1961 K. E. BRADWAY ETAL METHOD AND APPARATUS FOR CONTINUOUS COUNTERCURR PULPING OF' LIGNO-CELLULOSE MATERIALS 2 Sheets-Sheet 2 Filed May l2, 1959 PULP 70 S TURA GE SN R YN E02 U VA s m Mr .m m E@ Am s .d r/. W A w a 7 z A 1 W/ W T 0 n m Z P United States Patent O METHOD AND APPARATUS FOR CONTINUOUS COUNTERCURRENT PULPING OF LIGNO-CEL- LULOSE MATERIALS Keith E. Bradway, Franklin, and Albert P. Yundt, Southampton County, Va., assignors to Union Bag-Camp Paper Corporation, New York, N.Y., a corporation of Virginia Filed May 12, 1959, Ser. No. 812,681 Claims. (Cl. 162-17) The present invention relates to improvements in pape-rmaking. More particularly it pertains to an improved method of pnlping wood or other ligno-cellulose material in a continuous countercurrent manner and to produce a superior pulp with a saving in chemical and other costs.

In the method described the fresh pulping liquor first contacts the fully pulped product and does not reach the incoming untreated ligne-cellulose fragments until it becornes exhausted to the point of being spent liquor.

Herein wood will be used for illustrativeV purposes because it is the most commonly pulped ligno-cellulose material in the form of subdivided moisture containing ligno-cellulose fragments, but the description is not to be construed as confining the scope of the invention to wood alone. It will be apparent that the same principles apply to other ligno-cellulose materials such as bagasse and bamboo.

The present practice of the industry is to pulp concurrently. Pulping, considered as an extractive process, is more eiiiciently carried out countercurrently because the concentration of the extract can be readily brought to a maximum and the extractable residue in the product can be readily brought to a minimum.

Wood, in the form of chips, and cooking chemicals in the form of aqueous cooking liquor are placed together in a pressure vessel, either batch or continuous, and allowed to react at high temperature and superatmospheric pressure. The cooking liquor reacts with and dissolves the incrusting substances surrounding the fibers, turning the wood into a soft mass which may be separated more or less readily into its substituent fibers.

In pulping, the lastly removed lignin is less reactive and much more difficulty removable than the remainder. Hence it is desirable that the highest concentration of chemical be used at the end of the cook. Furthermore, the highly reactive lignin which is first removed from the wood consumes chemical so readily that the existing liquor can be very nearly exhausted of its reactive chemical content While eiiiciently extracting reactive lignin. These factors are readily and eiciently controlled by countercurrent digestion or cooking.

While many methods for countercurrent cooking of wood chips have been proposed, the most common methods employ a plurality of pressure vessels which are intermittently loaded with chips and each used for cooking with liquor blown from a cook which is one step further advanced in the cycle. These methods require a group of pumps and strainers for liquor removal from each pressure vessel which are costly to install and difficult to maintain. Operation of the battery of cooking pressure vessels requires synchronization of the operations of each of the digesters or else the additional use of accumulators for holding liquors at various stages of digestion. It can thus be seen that a continuous system in a single digester would permit more etiicient utilization of the countercurrent principle of digestion.

Many of the currently used and proposed continuous digesters are unsuitable for truly countercurrent flow of cooking liquor because these do not take into account the fact that a special cooker construction is necessary in order to prevent density differences from causing continuous mixing of the liquors which are supposedly held frice at varying degrees of exhaustion but which under these tests are found not to be controlled accurately for strength. The entering fresh liquor in the countercurrent system gradually increases in density as it dissolves increasing amounts of wood substance in its progression through the cooker. In the final stages of liquor treatment it begins to meet incoming fresh wood and some of the cooking chemicals are then absorptively tied up with the wood substances and hence is at least temporarily removed from `the spent solution. Additionally, the spent liquor is diluted by taking up of moisture from the incoming chips. Thus, the liquor begins to decrease in density. Accordingly, a feature of novelty of this invention resides in the discovery of the necessity in countercurrent cooking for having a cooker wherein the confined liquor enters at a high point, progresses downwardly under confinement as it increases in density, and then progresses upwardly under confinement as it decreases in density. Any other construction permitting free ow of liquor would result in mixing because of density differences.

In the method of continuous countercurrent digestion of subdivided moisture-containing ligno-cellulose fragments with lignin-removing aqueous cooking liquor at ligno-cellulose digestion temperatures and pressures which is employed in the present invention, these fragments such as wood chips, are first conveyed downwardly in a zone from a iirst elevated point at the fragment charging end in a tubular digester towards the lowest point in said digester. During this downward travel of the chips, the cooking liquor is progressively decreasing in density from said lowest point towards said rst elevated point due to absorption of cooking chemicals from the cooking liquor by the freshly introduced fragments, eg., fresh wood chips and replacement of these absorbed chemicals by water which is picked up from the fresh chips by the spent cooking liquor. As mentioned above, the lignin is more reactive in the fresh wood chips, the cooking liquor is weaker in delignifying power at the exit spent end of the cooking Zone and the chips effectively exchange lower density moisture for higher density chemicals and salts to lower the gravity of the spent liquor as to exits from the digester.

In the second conveying step in accordance with the invention, the ligno-cellulose fragments are conveyed from the lowest point in the tubular digester along a Zone which extends progressively upwardly to a second elevated point located at the discharge end of the digester. It is at this discharge end for the finished delignifed pulp that fresh full strength cooking liquor is introduced. Due to the dissolving action exhibited by the fresh full strength cooking liquor for chemically encrusted partially digested fragments of ligno--cellulose there is an exchange of soluble chemical materials from the encrusted fragments to the full strength liquor in countercurrent flow from the second elevated point to the lowest point of the digester. Accordingly, the fresh cooking liquor increases in density progressively along the zone `defined by the conveying path of the wood chips towards the lowest point of the digester.

Illustratively, a continuous countercurrent tubular digester of V-shaped cross-section for carrying out the method of the invention and as shown in the drawings can serve to introduce cooking liquor of fresh density value D0 at the delignified chip discharge end at the first elevated point in the digester where fresh cooking liquor enters at the left end as shown in the drawings. The density increases to the maximum value D max. at the lowest point of the zone in the digester for countercurrent treatment of liquor with chips in the downward travel from the left end towards the center as shown in the drawings.

Thereafter, in the second conveying path through the subsequent digestion zone for finishing delignification of the wood chips, the upward path is taken from the lowest point of the treating zone in the digester to the second elevated point at the charging at the right end of the digester (see drawings). At this point, fresh chips are introduced, and the density of the cooking liquor drops from the value of D max. which was exhibited at the lowest end of the digester to an intermediate value of density of spent liquor. This last value of density of spent liquor which is less than the density of the fresh cooking liquor and is significantly less than the maximum density at the common lowest point in the first and second conveying zones.

It is to be emphasized that the first and second conveying steps are carried out without any mixing or turbulence of cooking liquor in the two zones. To insure the improvement in efiicient utilization of cooking liquor in accordance with the invention, the countercurrent extraction by liquor of varying density must be assured.

The conveying movement of ligno-cellulose material through the digester is by means of screw advancement, the chips being conveyed first downwardly against the force of gravity by the conveyor flights in the first conveying zone and thereafter upwardly along the conveyor flights in the second conveying zone. By positioning the flights immediately adjacent the inner walls of the respective sections of the digester and by adjusting the nfiow of fresh cooking liquor and outflow of spent liquor in relation to the counterflow of wood chips to prevent turbulence and promote stratification in the digester there is soon realized a steady state condition in the digester of stratification of liquor, delignication and chemical consumption. This is evidenced by the achievement of maximum density at the lowest point of the digester, this point located by the intersecting paths of the conveyed chips in the first and second zones, respectively.

The conveying path in the first zone of digestion, this first zone representing the initial movement of the liquor from the left end to the right end of the tubular digester apparatus as shown in the drawings, in general, defines a straight line which intersects with the similar straight line for the second conveying movement. This first straight line makes an angle with the vertical line passing through the vertex which varies from about to about 85 to the vertical but which is preferable from about 30 to about 60. Similarly, the straight line passing through the second conveying path makes an angle with the vertical at the intersection between first and second conveying paths varying from about 15 to about 85, preferably 3060.

Within the tubular confines of the divergent screw conveyor housing for the passage of chips, the liquor under non-turbulent propulsion conditions readily stratifies to assume steady state density values under the effect of gravity.

By subjecting the wood chips to chemical pretreatment, particularly in the case of neutral sulfite pulping and bisulfite pulping procedures important advantages of reduced chemical consumption, reduced cooking time and improved product purity are achieved in digesting wood pulp by the method of the invention.

Predigestion is desirably carried out by preheating the wood chips with spent liquor or alkaline liquor under superatmospheric pressure at a temperature varying from about 120 C.-200 C. for about 5-30 minutes, the shorter time at higher temperatures. Thereafter digestion is effected at a temperature which is preferably above 160 C. and up to 195 C. for a period of about 1 to about 8 hours.

In the case of neutral sultite pulp digestion, predigestion is effected by merely soaking the chips in dilute sodium carbonate or dilute sodium hydroxide containing up to 5% alkali as sodium carbonate or sodium hydroxide. Thereafter by heating to about -170 C. suicient predigestion of the chips is achieved to provide a substantial chemical savings in the amount of chemicals concerned during digestion.

By buffering the fresh liquor to a pH value between 6.9 and 7.6 the process may be additionally varied to effective control of the acidity of the partly and completely spent liquor and thereby to enhance reaction of the spent liquid in its weaker strength stages with the pulp. Superatmospheric pressure varying from about 50 p.s.i.g. to about p.s.i.g. are utilized in order to achieve the optimum temperature for digestion.

Surprisingly, continuous operation is permitted even when pumping the spent cooking liquor at the wood chip charging end and feeding by pumping fresh liquor at a rate of up to 20 gallons per minute for through put of 25-30 pounds of wood chips per minute. The pulping provides highly efficient operation and results improved pulp color, shortened time of digestion, reduced organic solids in spent liquor. There results less damage to carbohydrate with resulting improved strength and yield by the process of the invention.

If density stratification is prevented by either inducing turbulence through either wide spacing of the edges of the screw flights from the inner wall of the tubular condenser and unduly rapid rotation of the screw or by forming incoming fresh liquor into the lowest point of the digester, then the new results of the invention will not be achieved. It is suiiicient that the clearance between screw ight edge and inner surface of the digester be from about Aa to 3A inch in the cross section of 4-12 inches or greater of digester as can be employed in order that the first condition be controlled. As to the latter condition of inducing turbulence by mixing fresh liquor, it is not serious for the preferred throughput of fresh chips illustrated in the examples and the pumping of fresh liquor may be easily controlled to prevent turbulence.

Other objects of the invention will in part be obvious and will in part appear hereinafter.

The invention accordingly comprises the several steps and the relation of one or more of such steps with respect to each of the others thereof, which will be exemplified in the method hereinafter disclosed, and the scope of the invention will be indicated in the claims.

For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawings, in which:

FIG. 1 illustrates a iiow sheet presenting a basic scheme for carrying out the method of the present invention, and

FIG. 2 shows a fiow sheet employed for carrying out such method.

Referring more particularly to the drawings, FIG. l shows that the chips or ligno-cellulosic material in some other appropriate fragmentary form are fed by means of feed mechanism yl to the continuous digester where they first enter downward section 2 and, at the appropriate point of the digestion, somewhere near the middle, begin moving upward in section 3, the advancement of chips in this illustration being accomplished by means of a screw. In the upper part of section 3, the chips drain free of excess cooking "liquor, then fall into discharge mechanism 4, and thence go to disintegrator 5. The disintegration step 5 may precede the discharge step 4 if desired or proceed simultaneously as in the normal blowing of fully cooked chemical pulps. The pulp then goes to washing system 6 where fresh water is added to recover the chemical contained within the chips as they left the digester. From the washing system the pulp is sent to storage and wash liquor is used to dissolve makeup chemical in liquor preparation system 7. The prepared liquor goes to storage 8 and is pumped by pump 9 to heater 10 and thence to the liquor inlet 11 of the digester. The liquor flows down section 3 of the digester, up through section 2 and is removed through strainer 12 and sent to storage for chemical recovery or is disposed of as a waste. Liquor removal rate is so adjusted as to maintain the liquor level in the digester and yet prevent its overflow with the exiting pulped material.

The cooker employed was four feet in diameter and forty feet long but Was especially modified with a liquor handling and discharge system as illustrated in the flow diagram of Fig. 2, now described in detail.

Rotary valve la was used to introduce the chips to the downward section 2a of the digester. The chips were carried down the digester by means of spaced flights attached to a flexible chain and moved upward through section 3a `of the digester by the same means. The chips fell out of the upper end of the digester into screw 19a feeding berizer 5a operating at full temperature and pressure from which the reiined pulp was discharged by means of restrictive blow valve 4a to blow tank 20a. The pulp in the blow tank was diluted with wash liquor and pumped to a Z-drum 4-stage Vacuum washing system 6a. Washed pulp was sent to storage. The wash liquors were used to dissolve makeup sodium sulfite in tank 7a, and the liquor was pumped to storage 8a. This storage is desirable in order to even out unavoidable minor density fluctuations in the cooking liquor. The `liquor was fed by means of pump 9a to point 10a where direct injection into a steam line was used to heat the liquor to the cooking temperature. The liquor was injected into cyclone 18a to separate the steam and liquor. The excess steam was sent to the digester to maintain pressure and to a series of external steam pipes running the length of the cooker to protect the digester from temperature drops caused by radiant and convective heat losses. The liquor entered the digester at port 11a, owed downward through section 3a, thence upward through section 2a, and was removed through strainer plate 12a.

Instead of feeding the fresh chips directly to this digester, it was more convenient in our particular setup to introduce them by means of rotary valve 13a to the existing installed horizontal tube screw feed pre-heater section 14a. Sloping baffle 16a was placed at the chip discharge end of the pre-heater so that the chips could be forced up, over, and discharged to the rotary valve 1a feeding the main digester, yet permitting the retention of sufficient liquor in the preheater to submerge the chips. The spent liquor from the main digester was relieved into the chip discharge end of the pre-heater at point 15a. It was discharged at the opposite end via strainer plate 17a and sent to a kraft process recovery system under its own pressure. Liquor levels in both vessels were maintained by means of external oat type regulators 21a and 22a. The normal pocket and leakage steam of rotary valve 1a was suiiicient to maintain the pre-heater at approximately half the main digester pressure. Chip retention time in the preheater was about fifteen minutes.

The use of such a pre-heater is not mandatory in the practice of this invention, but it has several advantages. It acts as an air removal stage and saves steam in that the heat utilized would otherwise be wasted. The use of two rotary valves in series reduces steam losses and increases the life of the valves. It serves as a preimpregnation stage of sorts so that sorne cooking chemical is present in the chips before full temperature is reached. -In the absence of such chemical, heat causes the lignin to become slightly more difficult to pulp. Most importantly, -it allows hydrolysis and removal of the combined wood acids at a lower temperature. They are largely washed out and can even be neutralized before full cooking temperature is reached. In the absence of neutralization, the wood acids are at least partially buffered by the cooking chemicals. This would be expected to signicantly decrease the degradation of the carbohydrates.

In conventional neutral suliite pulping it is general prac- 6 tice to add sodium carbonate or sodium bicarbonate as a buffering agent to prevent excessive acidity in cooking. As an example of this, a small-scale batch cook was made on gum chips using the following conditions:

Percent sodium sulfite 23 Percent sodium bicarbonate 3 Liquor to. wood ratio 5.0 Initial pH 8.5 Maximum temperature C 170 Time to max. temperature hours l Time at max. temperature do 5 At the end of the cook, the liquor was at pH 7.3 and contained 3% residual sodium sulfite. The pulp properties were:

Yield percent 62 Lignin content do 10.0

0.5% cupriethylenediamine viscosity centipoise 18.6

As an example of the use of this invention, hardwood chips, predominately gum, were pulped in the apparatus described. At the start up of the cook, the digester was filled with liquor originally containing grams per liter of sodium sullite which was pre-heated with direct steam to the cooking temperature. Liquor addition was stopped and chips were fed to the digester at a rate of 20.5 lbs. (oven dry basis) per minute. After three hours the cooked pulp began to come through to the washer. The cooking liquor iiow was resumed at the rate of 5.5 gallons per minute and the digester was operated continuously for a total of l2 hours at 120 p.s.i.g. steam pressure. At the end of this time, a steady state condition of chemical consumption obtained. Pulp of 8.5% lignin content was obtained at a yield of approximately 60% and with a 0.5% cupriethylenediamine viscosity of 30.9 cp. The chemical added to the digester was 17.7% based on the wood entering. The pulp blown to the washing system contained 3.5% sodium sulfite based on the wood entering. This was recovered in the washing system; the wash liquors would be used to prepare fresh cooking liquor. The net chemical required was 14.2% NaZSOg. This pulp was screened and 50 pounds 24 x 36"-500 count basis weight handsheets were prepared after beating in a Valley beater for strength determination.

At 5 00 cc. Canadian standard freeness the pulp strength was:

Samples of the cooking liquor were obtained entering the digester, at the liquor line of the pre-heater, and at sampling points midway on the upper and lower sides of the digester. The sample value provided at the bottom of the digester was not usable.

The liquors analyzed as follows:

These liquors had flashed off steam during collection and are stronger than the liquors in the digester, but their specific gravities would retain the same relative relationships.

EXAMPLE 2 Wood as in Example 1 was fed to the pre-heater at 16.4 lbs. per minute and cooking liquor containing 99 grams per liter of sodium sulite was pumped in at the rate of 4.5 gallons per minute. Cooling time was 3 hours at 120 p.s.i.g. steam pressure as in Example 1.

The pulp produced was obtained in `approximately 60% yield with a lignin content of 9% and with a 0.5% cupri- No specilic gravity data were obtained. It may be noticed that despite the fact that no additional buffer was used the pH remained above 7 except for the very short time near the liquor exit from the digester. The chemical pumped to the digester was 22.6% of the wood; 4.1% was discharged with the pulp and recovered in the washing system. The net chemical used was 18.5%. This pulp was screened in the laboratory and bleached in a three stage sequence of chlorine, caustic extraction, and chlorine dioxide. A strength evaluation was made after heating in the Valley beater. At 500 cc. Canadian standard freeness the strength properties were:

Pine chips which had been presoaked in sodium carbonate solution and containing approximately 2% sodium carbonate were fed to the pre-heater at a rate of 33.1 lbs. per minute. The cooking time was shortened to 11/2 hours and the pressure maintained at 120 lbs. gage. Cooking liquor containing a mixture of sodium bisulfite and sodium sultite at a pH of 7.0 and a concentration, calculated as sodium sulfite, of 114 grams per liter was pumped to the digester at the rate of 6.5 gallons per minute. As the cooking liquor analyses show, the liquor rate or concentration should have been lowered for minimum chemical consumption. The chemical added was 18.6%, that discharged with the pulp and recovered 53%; 0.1' a net feed of 12.8% as sodium sulte. Cooking liquid analyses were as follows:

Percent pH Sp.

NazSOg Grav.

Liquor to Pre-heater 44 7. 2 1. 094 Sample Point at Middle of Upper Side 55 7.1 1.100 Sample from Bottom of Cooker 65 7. 1 1.101 Sample Point at Middle of Lower Side 75 7. 2 1. 007

The maximum in specific gravity of the liquor can again be observed although in this case the presence of large excesses of sodium sulte makes the difference less noticeable. The pH data illustrate that with this system the pH could be kept nearly constant throughout the cook. The pulp produced was obtained in about 75% yield at a lignin content of 13% and with a 0.5% cupriethylenediamine viscosity of 20.5 cp. It was not as Well deiibered as the gum pnlps after the single stage of rening. A sample was subjected to a secondary debering operation in a Valley laboratory beater, caustic extraction and chlorine dioxide to 70 G.E. brightness and evaluated for strength properties at 500 cc. Canadian standard freeness with the following results:

From the above examples it is readily apparent that the practice of this invention results in considerable saving in cooking chemical over concurrent continuous or batch pulping. There are other advantages which are not so obvious. Recovery of cooking chemicals from the exhausted cooking liquor usually requires concentration by evaporation and burning of the contained organic matter. With concurrent pulping one must evaporate not only the water used for making the original cooking liquor but also that part of the pulp washing water used which does not exit with the washed pulp. With our invention, however, both these Waters are one and the same. Hence, there is a great saving in evaporation costs and more thorough washing of the pulp becomes economically possible. Additionally, most of the soluble organic matter has been removed from the pulp by the countercurrent liquor ilow before the pulp reaches the washer. For any degree of Washing eiiiciency, the less organic matter there is to be washed out, the less there will be left in the washed pulp. Hence steam pollution from this residue is greatly decreased in the practice of our invention. Because of the decreased chemical consumption, the ration of organic to inorganic solids in the liquor is increased and it burns better in the furnace.

Because of the large quality of cooking chemical that moves past any one particle in the cooker, more uniform cooking results. Particles having an exceptionally high chemical demand such as bark, rot, and other organic dirt will be completely pulped. The net result is a much cleaner pulp. Because of the somewhat shortened cooking time at a given temperature and because of easier control of pH, there can be less damage to the carbohydrate matter with resulting improved strength and yield.

Because uncooked chips are in contact with the liquor removal strainer plate rather than pulped ones as in the case of concurrent pulping, there is less diculty from strainer plate clogging.

The examples given and the discussion of the invention have been confined to neutral sulte pulping only because the countercurrent scheme is especially advantageous with this process. The invention is not limited to neutral sulfite pulping however but may be applied to other pres- 9 ently used and proposed pulping process, especially bisuliite pulping.

Since certain changes may be made in carrying out the above method without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

Having described our invention, what we claim as new and desire to secure by Letters Patent is:

1. A method for continuous countercurrent digesting of subdivided moisture containing ligno-cellulose fragments comprising premixing said ligne-cellulose fragments with alkaline liquid selected from the group consisting of dilute alkali containing up to of alkali and spent liquor from the process, pre-heating to cooking temperature of between 120-200 C. for 5 to 30 minutes prior to its introduction into the tubular digester, said pre-heating step serving to eliminate occluded gases in said liquid ligno-cellulose fragments and permitting release of acids contained in said ligno-cellulose fragments at lower cooking temperatures without substantial deligniication in the pre-heating step, flowing lignin-removing aqueous cooking liquor at digesting temperature countercurrent to said fragments, withdrawing spent cooking liquor at the fresh ligne-cellulose fragment charging end of a tubular digester and discharging diligniiied fragments from the discharge end opposite said fresh fragment charging end of said tubular digester and at which discharging end the fresh cooking liquor is introduced, said ligno-cellulose fragments being conveyed in said digester in a zone extending progressively downwardly from a iirst elevated point in said digester at the charging end to the lowest point in said digester, and said cooking liquor progressively decreasing in density from said lowest point towards said rst elevated point and said fragments being conveyed thereafter in said digester in a zone extending progressively upwardly from said lowest point to a second elevated point at the discharge end of said digester While said cooking liquor is progressively increasing in density from said second elevated point towards said lowest point while preventing turbulent mixing of cooking liquor from said iirst downward conveying step and said second upward conveying step whereby the cooking liquor at the lowest point in said digester is maintained at the maximum density of working liquor used in said digester and discharging deligniiied cellulose particles at said second elevated point from said digester.

2. A method as claimed in claim 1 wherein said ligno cellulose fragments are wood chips and are pre-heated for a period of about 5 to 30 minutes to a temperature of from about C. to about 200 C. whereafter said digestion is carried out at elevated pressure and a temperature of about C. to about 190 C. for a period of about 1 to 8 hours in the presence of a buffering agent to adjust the acidity of the spent liquor to a pH value of between about 6.9 and 7.6.

3. A method as claimed in claim 2 wherein said digestion is carried out under steam pressure varying from about 50 p.s.i.g. to about 180 p.s.i.g.

4. A method as claimed in claim 3 wherein said digestion is carried out at a temperature of about -180 C. for a period of time of about 11/2 hours under a pressure of about 120 p.s.i.g.

5. A method as claimed in claim 3 wherein said wood chips are pine chips which have been pre-soaked in an aqueous solution containing 2% sodium carbonate and in which said cooking liquor is a neutral solution containing sodium bisuliite and sodium sullite at a concentration of about 1.00 to about 120 grams per liter, the ratio of liquor to wood being between 4:1 to 6:1 whereby there is produced a finished pulp having a lignin content of about 6% to about 11%.

References Cited in the file of this patent UNITED STATES PATENTS 448,581 Kaffenberger Mar. 17, 1891 2,359,543 Branzell Oct. 3, 1944 FOREIGN PATENTS 1,096,833 France Feb. 9, 1955 OTHER REFERENCES Paper Trade Journal, pages 76, 77, Nov. 6, 1947. 

1. A METHOD FOR CONTINUOUS COUNTERCURRENT DIGESTING OF SUBDIVIDED MOISTURE CONTAINING LIGNO-CELLULOSE FRAGMENTS COMPRISING PREMIXING SAID LIGNO-CELLULOSE FRAGMENTS WITH ALKALINE LIQUID SELECTED FROM THE GROUP CONSISTING OF DILUTE ALKALI CONTAINING UP TO 5% OF ALKALI AND SPENT LIQUOR FROM THE PROCESS, PRE-HEATING TO COOKING TEMPERATURE OF BETWEEN 120-200*C. FOR 5 TO 30 MINUTES PRIOR TO ITS INTRODUCTION INTO THE TUBULAR DIGESTER, SAID PRE-HEATING STEP SERVING TO ELIMINATE OCCLUDED GASES IN SAID LIQUID LIGNO-CELLULOSE FRAGMENTS AND PERMITTING RELEASE OF ACIDS CONTAINED IN SAID LIGNO-CELLULOSE FRAGMENTS AT LOWER COOKING TEMPERATURES WITHOUT SUBSTANTIAL DELIGNIFICATION IN THE PRE-HEATING STEP, FLOWING LIGNIN-REMOVING AQUEOUS COOKING LIQUOR AT DIGESTING TEMPERATURE COUNTERCURRENT TO SAID FRAGMENTS, WITHDRAWING SPENT COOKING LIQUOR AT THE FRESH LIGNO-CELLULOSE FRAGMENT CHARGING END OF A TUBULAR DIGESTER AND DISCHARGING DILIGNIFIED FRAGMENTS FROM THE DISCHARGE END OPPOSITE SAID FRESH FRAGMENT CHARGING END OF SAID TUBULAR DIGESTER AND AT WHICH DISCHARGING END THE 